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There is a hidden setting for this. It would be great if it were made easier to discover and set. If you want to test it and see whether it does what you're looking for, here's how:
(A) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.
More info on about:config: Configuration Editor for Firefox. Please keep in mind that changes made through this back door aren't fully supported and aren't guaranteed to continue working in the future.
(B) In the search box in the page, type or paste browser.tabs.loadBookmarksInTabs and pause while the list is filtered
(C) Double-click the preference to switch the value from false to true
To open a bookmark in a new tab, you can click on it with the Ctrl key pressed (command on Mac), or just click with the mouse wheel.
Do the same to open a History item in a new tab.
In `about:config` set `browser.tabs.loadBookmarksInTabs` to `true`
Now when you click on a bookmark it should open in a new tab.
Hope it helps.
Thanks, that works and does the job I need. The only thing I need now is how to open a new tab in an individual bookmark.